iio: fix kernel-doc build errors

Fix build errors in kernel-doc notation. Symbols that end in '_'
have a special meaning, but adding a '*' makes them OK.

../drivers/iio/industrialio-core.c:635: ERROR: Unknown target name: "iio_val".
../drivers/iio/industrialio-core.c:642: ERROR: Unknown target name: "iio_val".

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/industrialio-core.c

@@ -631,15 +631,15 @@ static ssize_t __iio_format_value(char *buf, size_t len, unsigned int type,
  * iio_format_value() - Formats a IIO value into its string representation
  * @buf:	The buffer to which the formatted value gets written
  *		which is assumed to be big enough (i.e. PAGE_SIZE).
- * @type:	One of the IIO_VAL_... constants. This decides how the val
+ * @type:	One of the IIO_VAL_* constants. This decides how the val
  *		and val2 parameters are formatted.
  * @size:	Number of IIO value entries contained in vals
  * @vals:	Pointer to the values, exact meaning depends on the
  *		type parameter.
  *
  * Return: 0 by default, a negative number on failure or the
  *	   total number of characters written for a type that belongs
- *	   to the IIO_VAL_... constant.
+ *	   to the IIO_VAL_* constant.
  */
 ssize_t iio_format_value(char *buf, unsigned int type, int size, int *vals)
 {